<!DOCTYPE html>
<html>
<head lang="en">
    <meta charset="UTF-8">
    <title></title>
    <style>
        *{
            margin: 0;
            padding: 0;
            list-style: none;
        }
        li:nth-child(1){
            width: 200px;
            height: 50px;
            background-color: red;
            float: left;
        }
        li:nth-child(2){
            width: calc(100% - 200px);
            height: 50px;
            background-color: yellow;
            float: left;
        }
    </style>
</head>
<body>
<ul>
    <li>1</li>
    <li>2</li>
</ul>
</body>
</html>